1. Field of the Invention
This invention relates to aqueous binders which can be formulated into coating compositions for any substrates, especially rigid substrates, yielding soft-feel coatings thereon.
2. Descirpition of the Prior Art
In the coating of hard substrates, such as plastics, there is frequently a desire for the coating to achieve a soft feel, where the coated surface, although feeling soft to the touch (like skin or leather), is unimpaired in terms of its mechanical and chemical resistance in comparison to the conventional coatings.
The so-called soft feel coating materials achieve this objective. The substrates coated with them have Kxc3x6nig pendulum hardnesses of less than 20 seconds for coating films approximately 50 xcexcm in thickness. The surfaces are scratch resistant (scratch test with the fingernail) where scratching leaves a trace, it can be removed again simply by wiping with the fingertip. Without sticking or being tacky, the coated surfaces are slip resistant; for example, a glass or metal block does not slip on a surface coated with a soft feel coating material but instead tips when the angle of inclination is gradually increased. If a coating film is detached from a glass surface coated with it, and its extension is measured, a reversible extension of up to 500% is found on measurement at room temperature with a stretching rate of 0.6 cm/s.
Binders for these soft feel coating materials preferably have segments of polymers having a low glass transition temperature; the crosslinking density must still be high enough for there to be very little or no reduction at all in the mechanical and chemical resistance properties but low enough to allow the effect of the low glass transition temperature to be manifested macroscopically as well.
Two-component, or two-pack binders have been disclosed for soft feel coating materials that are based on hydroxyl-containing polyester-urethanes, which are to be cured preferably with isocyanates but also, for example, with acid anhydrides or amino resins.
It has proven impossible to date to provide practicable one-component, or one-pack binders for soft feel coating materials. Although crosslinking with amino resins may be effected at elevated temperature in the manner of a one-component system, the soft surface of the soft feel coating is adversely affected by the high temperatures required. Especially when coating plastics, the crosslinking temperature is limited by the softening temperature of the polymer substrate.
The known, polyisocyanate-based systems which cure at room temperature share the typical problems of isocyanate-cured systems: the limited pot life and limited acceptability from the standpoint of occupational hygiene. A further important disadvantage of isocyanate curing agents in conjunction with soft feel coating materials is the xe2x80x9caftercuringxe2x80x9d which is observed. The polyfunctional isocyanates used for the crosslinking are usually employed in excess. They are able to react with water, or even with the atmospheric moisture, and do so with decarboxylation and formation of the corresponding amines, which in turn form ureas with isocyanates that have not yet reacted. This reaction occurs during the drying of the coating material and adversely affects the feel of the coating film, which is still soft directly following application and initial crosslinking.
There was therefore a need to develop a binder for soft feel coating materials which durably retains this soft feel. A further object was to provide a one-component binder which cures even at moderate temperatures (above 70xc2x0 C., in particular from about 80 to about 150xc2x0 C.) and yet imposes very little restriction on the pot life.
This object has been achieved by the provision of a reactive combination comprising a hydroxyl-containing polyester-urethane and a melamine resin.
The invention accordingly provides a water dilutable binder for soft feel coating materials, comprising an aqueous dispersion of a hydroxyl-containing polyester-urethane A, obtainable by reacting a polyester A1 with hydroxyl groups as functional groups, said polyester A1 having been synthesised from linear, branched or cyclic aliphatic or aromatic difunctional or polyfunctional carboxylic acids A11 and aliphatic linear or branched difunctional or polyfunctional alcohols A12 as reactants, with a polyfunctional isocyanate A2, a compound A3 having at least one hydroxyl, mercapto or amino group which is reactive toward isocyanates and having at least one acid group which reacts only to a minor extent if at all with isocyanates, and also, optionally, with a drying or, preferably, nondrying oil A4, an aqueous solution or dispersion of a triazine resin B which for each molecule derived from a triazine has between 2. and 6 groups selected from methylene groups, methylene ether groups, methylol groups and alkoxymethyl groups, wherein the glass transition temperature of the polyester-urethane A is between xe2x88x9270 and xe2x88x9220xc2x0 C., preferably from xe2x88x9260 to xe2x88x9225xc2x0 C., and in particular from xe2x88x9250 to xe2x88x9230xc2x0 C., and wherein the hydroxyl number of the polyester-urethane A is from 10 to 120 mg/g, preferably from about 20 to about 100, and in particular from 25 to 95 mg/g.
The term xe2x80x9cminor extentxe2x80x9d as used herein means that, under otherwise identical reaction conditions, the reaction rate of the isocyanate-reactive hydroxyl, mercapto or amino groups is at least 5 times as great as that of the acid groups with respect to the same compound containing isocyanate groups.
The polyester A1 is prepared preferably in two stages in a known manner by condensing the entirety of the carboxylic acid component A11 with at least a portion A121 of the polyol component A12, the average functionality of A121 in the first stage being between 1.8 and 2.3, preferably from 1.9 to 2.1.
As component A11, it is preferred to use dicarboxylic acids having 4 to 40 carbon atoms, particular preference being given to succinic acid, adipic acid, hexahydrophthalic acid, cyclohexanedicarboxylic acid, and a mixture of branched aliphatic dicarboxylic acids obtainable by dimerising unsaturated fatty acids. Up to 10%, preferably up to 5%, of the mass of A11 may comprise one or more higher carboxylic acids having three or more carboxylic acid groups per molecule, such as tricarboxylic and tetracarboxylic acids.
As component A121, it is preferred to use difunctional alcohols having 2 to 20 carbon atoms. Glycol, 1,2- and 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, and cyclohexanedimethanol are especially suitable, as are the ether alcohols, diethylene glycol, triethylene glycol and higher oxyethylene diol oligomers up to a degree of polymerisation of approximately 10. It is also possible to use oligomeric propylene glycols; the mass fraction of the ether alcohols in component A121 is preferably up to 30% of the mass of component A121. In the context of the invention it is also possible to use alcohols having a functionality of three or more, and monofunctional alcohols, in each case individually or together, as constituents of component A121. In that case, the amount of these alcohols of higher or lower functionality must be chosen such that the average functionality satisfies the above condition.
Following the condensation of the preferably difunctional reactants of the first stage, the resulting polyester, which typically has an acid number of less than 10 mg/g, preferably less than 5 mg/g, is transesterified with a trifunctional or higher polyfunctional aliphatic alcohol or with a mixture of two or more aliphatic linear or branched alcohols having an average functionality of from 2.2 to 4.5, preferably from 2.5 to 4.0, and in particular from 2.8 to 3.8, referred to collectively below as A122. The component A122 preferably comprises trifunctional alcohols such as glycerol, trimethylolpropane and trimethylolethane, tetrafunctional alcohols such as pentaerythritol, ditrimethylolethane and ditrimethylolpropane, which in accordance with the invention are used preferably in a mixture with difunctional alcohols as defined under A121,and also alcohols of higher functionality such as mannitol, sorbitol and dipentaerythritol. Following the transesterification step, the resultant branched polyester A1 should have a hydroxyl number of from about 50 to about 150 mg/g, preferably from 70 to 130, and in particular from 80 to 120 mg/g, and an acid number of less than 8, in particular less than 5 mg/g.
The polyfunctional isocyanates A2 are aliphatic or aromatic compounds having (on average, where mixtures of two or more compounds are used) two or more than two isocyanate groups per molecule. Suitable aromatic diisocyanates include tolylene diisocyanate (technical-grade mixture of the 2,4- and the 2,6-isomers) and diphenylmethane diisocyanate. Preference is given to linear, branched and cyclic aliphatic isocyanates, especially difunctional isocyanates, having 2 to 15 carbon atoms in the (cyclo)aliphatic radical. Among the aromatic isocyanates, it is also possible to use, preferably, those which carry isocyanate groups on a carbon atom having an aliphatic character; one example thereof is tetramethylxylylene diisocyanate. Other preferred isocyanates are 1,6-diisocyanatohexane, 1,6-diisocyanato-3,3,5- and -3,5,5-trimethylhexane, 1,4-diisocyanatocyclohexane, isophorone diisocyanate, 2,2-bis(4-isocyanatocyclohexyl)propane, and also the uret diones, allophanates and biurets derived therefrom.
The compounds A3 have at least one, preferably two or more, especially two, groups which undergo addition reaction with isocyanates to form urethanes, ureas or thiocarbamates, i.e., hydroxyl, amino, or mercapto groups. As a further functional group they carry an acid group which even under the chosen reaction conditions undergoes little or no reaction with the isocyanate. The acid groups are selected from organically bonded carboxylic, sulfonic and phosphonic acid groups. Preferably, the acid group is sterically shielded by adjacent substituents. Particular preference is given to bishydroxymethylalkanoic acids such as dimethylolpropionic acid, dimethylolbutyric acid, and dimethylolacetic acid.
The oils A4 are the known esters of fatty acids and glycerol, such as linseed oil, and safflower oil; preference is given to nondrying oils, including in particular those in which at least some, preferably more than 5%, of the mass of the fatty acids is also hydroxy-functional. Particular preference is given to castor oil.
Preferably, the polyester-urethane A has an acid number of from about 10 to about 60, with particular preference from about 12 to about 50, and in particular from about 15 to about 40 mg/g, and the acid groups of the polyester-urethane are neutralised to the extent of from 20 to 100%, with particular preference from 30 to 90%, and in particular from 40 to 80%.
The hydroxyl number is defined in accordance with DIN 53 240 as the ratio of that mass mKOH of potassium hydroxide having exactly the same number of hydroxyl groups as a sample under analysis to the mass mB of this sample (mass of the solid in the sample in the case of solutions or dispersions); its customary unit is xe2x80x9cmg/gxe2x80x9d.
The acid number is defined in accordance with DIN 53 402 as the ratio of that mass mKOH of potassium hydroxide required to neutralise a sample under analysis to the mass mB of this sample (mass of the solid in the sample in the case of solutions or dispersions); its customary unit is xe2x80x9cmg/gxe2x80x9d.
The resins used as component B are partially or fully etherified triazine resins, derived from melamine, acetoguanamine, caprinoguanamine, and benzoguanamine or their mixtures. For heat-sensitive substrates it is preferred to use melamine resins in which the number of methylol groups in relation to the number of groups which have been etherified or condensed further to methylene, methylene ether or alkoxymethyl groups is sufficient for the reactivity.
Methylol or N-methylol groups are groups of the type  greater than Nxe2x80x94CH2OH, methylene groups are groups of the type  greater than Nxe2x80x94CH2xe2x80x94N less than , methylene ether groups are groups of the type  greater than Nxe2x80x94CH2xe2x80x94Oxe2x80x94CH2xe2x80x94N less than , and alkoxymethyl groups are groups of the type  greater than Nxe2x80x94CH2xe2x80x94Oxe2x80x94R, where R is usually an alkyl radical selected from methyl, ethyl, n- and iso-propyl and also n-, iso-, sec- and tert-butyl groups, preference being given to methyl and the n- and iso-butyl groups.
Where plastics, textiles, leather or wood are to be coated with the soft feel coating materials of the invention, it is preferred to use those triazine resins which contain predominantly N-methylol groups. Such resins undergo crosslinking reaction with the polyester-urethane even at relatively low temperatures above about 80xc2x0 C. In the case of the coating of metals or glass, it is also possible to use the slower-reacting etherified melamine resins, with or without the addition of customary acidic catalysts.
Predominantly as used herein means that, in this case, the number of N-methylol groups is higher than the number of any other substituent group, such as methylene, alkoxymethyl, or methylene ether groups, in an amino functionality of the triazine resin.
The amounts of components A and B are advantageously chosen so that the ratio of the number of hydroxyl groups in A to the number of reactive groups in B is from 0.7:1 to 1.5:1, preferably from 0.8:1 to 1.4:1, and with particular preference from 0.9:1 to 1.3:1.
The system of the invention cures, surprisingly, from as low as about 80xc2x0 C., preferably from about 90xc2x0 C. Relative to the known systems curable with polyfunctional isocyanates, the systems of the invention have the following advantages:
At approximately equal curing temperature (above 80xc2x0 C.), the systems of the invention are dust-dry after just a short period (about 0.5 hour) of drying in air; this dust-dry coat may immediately be recoated without the occurrence of the xe2x80x9cpoppingxe2x80x9d known with isocyanate-curing systems. In contrast to the isocyanates, the melamine resins in conjunction with the polyester-urethanes are not used in excess; instead, a stoichiometric amount (one methylene, methylol or alkoxymethyl group per hydroxyl group of the polyester-urethane resin) of the melamine resin is sufficient for curing.
At coat thicknesses of as little as above about 30 xcexcm, a soft feel may be obtained with the system of the invention; in the case of the known isocyanate-curing systems, coat thicknesses of at least 50 xcexcm are required. In contrast to the known isocyanate-curing systems, the soft feel coating materials of the invention do not exhibit the phenomenon of aftercuring; the soft feel does not gradually disappear but instead is retained for a long period.
The binders of the invention are outstandingly suitable for the formulation of coating materials which give the substrates coated with them the above-described soft feel. They are used in particular to coat metals, plastics, glass, ceramic, textiles, leather, and wood.